1. Field
Example embodiments relate to neutron monitoring systems including gamma thermometers and methods of calibrating nuclear instruments using gamma thermometers. Also, example embodiments relate to neutron monitoring systems including gamma thermometers in which nuclear instruments of the neutron monitoring systems are calibrated using compensated signals from the gamma thermometers. Additionally, example embodiments relate to methods of calibrating nuclear instruments using compensated signals from the gamma thermometers.
2. Description of Related Art
FIG. 1 is a sectional view, with parts cut away, of reactor pressure vessel (“RPV”) 100 in a related art boiling water reactor (“BWR”). As known to a person having ordinary skill in the art (“PHOSITA”), during operation of the BWR, coolant water circulating inside RPV 100 is heated by nuclear fission produced in core 102. Feedwater is admitted into RPV 100 via feedwater inlet 104 and feedwater sparger 106 (a ring-shaped pipe that includes apertures for circumferentially distributing the feedwater inside RPV 100). The feedwater from feedwater sparger 106 flows down through downcomer annulus 108 (an annular region between RPV 100 and core shroud 110).
Core shroud 110 is a stainless steel cylinder that surrounds core 102. Core 102 includes a multiplicity of fuel bundle assemblies 112 (two 2×2 arrays, for example, are shown in FIG. 1). Each array of fuel bundle assemblies 112 is supported at or near its top by top guide 114 and at or near its bottom by core plate 116. Top guide 114 provides lateral support for the top of fuel bundle assemblies 112 and maintains correct fuel-channel spacing to permit control rod insertion.
The coolant water flows downward through downcomer annulus 108 and into core lower plenum 118. The coolant water in core lower plenum 118 in turn flows up through core 102. The coolant water enters fuel assemblies 112, wherein a boiling boundary layer is established. A mixture of water and steam exits core 102 and enters core upper plenum 120 under shroud head 122. Core upper plenum 120 provides standoff between the steam-water mixture exiting core 102 and entering standpipes 124. Standpipes 124 are disposed atop shroud head 122 and in fluid communication with core upper plenum 120.
The steam-water mixture flows through standpipes 124 and enters steam separators 126 (which may be, for example, of the axial-flow, centrifugal type). Steam separators 126 substantially separate the steam-water mixture into liquid water and steam. The separated liquid water mixes with feedwater in mixing plenum 128. This mixture then returns to core 102 via downcomer annulus 108. The separated steam passes through steam dryers 130 and enters steam dome 132. The dried steam is withdrawn from RPV 100 via steam outlet 134 for use in turbines and other equipment (not shown).
The BWR also includes a coolant recirculation system that provides the forced convection flow through core 102 necessary to attain the required power density. A portion of the water is sucked from the lower end of downcomer annulus 108 via recirculation water outlet 136 and forced by a centrifugal recirculation pump (not shown) into a plurality of jet pump assemblies 138 (only one of which is shown) via recirculation water inlets 140. Jet pump assemblies 138 are circumferentially distributed around core shroud 110 and provide the required reactor core flow.
As shown in FIG. 1, a related art jet pump assembly 138 includes a pair of inlet mixers 142. A related art BWR includes 16 to 24 inlet mixers 142. Each inlet mixer 142 has an elbow 144 welded to it that receives water from a recirculation pump (not shown) via inlet riser 146. An example inlet mixer 142 includes a set of five nozzles circumferentially distributed at equal angles about the axis of inlet mixer 142. Each nozzle is tapered radially inwardly at its outlet. Jet pump assembly 138 is energized by these convergent nozzles. Five secondary inlet openings are radially outside of the nozzle exits. Therefore, as jets of water exit the nozzles, water from downcomer annulus 108 is drawn into inlet mixer 142 via the secondary inlet openings, where it is mixed with coolant water from the recirculation pump. The coolant water then flows into jet pump assembly 138.
FIG. 2 is a top plan view of a related art core 200. As known to a PHOSITA, core 200 may include fuel bundles 202, peripheral fuel bundles 204, and/or control rods 206. Two or more of fuel bundles 202 may be included in fuel bundle assemblies 208. Core 200 may include, for example, hundreds or thousands of fuel bundles 202 and/or tens or hundreds of peripheral fuel bundles 204. As shown in FIG. 2, for example, core 200 may include approximately one thousand and twenty-eight (1,028) fuel bundles 202, approximately one hundred and four (104) peripheral fuel bundles 204, and/or approximately two hundred and sixty-nine (269) control rods 206.
The distribution of fuel bundles 202, peripheral fuel bundles 204, and/or control rods 206 in core 200 may or may not be symmetric. Additionally, if symmetry exists, it may include one or more of mirror-image symmetry, diagonal symmetry, rotational symmetry, translational symmetry, quadrant symmetry, and octant symmetry. As shown in FIG. 2, for example, one or more control rods 206 may be disposed in or near a geometric center of core 200.
Core 200 also may include one or more types of neutron monitors. These monitors may include, for example, one or more source range monitors, one or more intermediate range monitors, and/or one or more power range monitors. In a related art BWR, the one or more source range monitors may be fixed or movable. Similarly, in a related art BWR, the one or more intermediate range monitors may be fixed or movable.
At least some of the overall range of a related art source range monitor and/or a related art intermediate range monitor may be covered by a startup range neutron monitor (“SRNM”) or wide range neutron monitor (“WRNM”). Similarly, at least some of the overall range of a related art intermediate range monitor and/or a related art power range monitor may be covered by a local power range monitor (“LPRM”). In a related art BWR, the SRNMs and/or the LPRMs may be fixed.
Core 200 may include, for example, tens of SRNM detectors and/or tens or hundreds of LPRM detectors. Although not shown in FIG. 2, core 200 may include, for example, approximately twelve (12) SRNM detectors. As shown in FIG. 2, for example, core 200 may include approximately two hundred and fifty-six (256) LPRM detectors in approximately sixty-four (64) LPRM assemblies 210. For example, one or more LPRM assemblies 210 may include four LPRM detectors (i.e., each LPRM assembly 210 may include four LPRM detectors).
FIG. 3 is a perspective view, partly broken away, showing a structure of a related art gamma thermometer (“GT”) assembly 300. FIG. 4 is a view showing a principle for measuring a gamma ray heating value of GT assembly 300.
As known to a PHOSITA and as discussed, for example, in U.S. Pat. No. 6,310,929 B1 (“the '929 patent”) and U.S. Pat. No. 6,408,041 B2 (“the '041 patent”), GT assembly 300 may include a thin and long rod-like assembly having a length substantially covering an effective fuel length of core 200 (e.g., between about 3 m and about 5 m in an axial direction of core 200. The equations and associated explanations of the '929 patent and the '041 patent are incorporated herein by reference.
As shown in FIG. 3, GT assembly 300 may include cover tube 302 and core tube 304. Annular space portions 306 may be formed between cover tube 302 and core tube 304. Each annular space portion 306 may form an adiabatic portion of GT assembly 300. For that purpose, annular space portions 306 may be filled with a gas having a low heat conductivity, such as argon (or another inert gas) or nitrogen. GT assembly 300 may include four or more annular space portions 306 (e.g., eight or nine). Annular space portions 306 may be discretely arranged at equal intervals in an axial direction of GT assembly 300.
Core tube 304 may have an internal hole 308 (see FIG. 4) that may extend through a center portion of core tube 304 along an axial direction of core tube 304. Cable sensor assembly 310 may be fixed inside internal hole 308. Cable sensor assembly 310 may include built-in heater 312, plurality of differential-type thermocouples 314, and cladding tube 316. Built-in heater 312 may function as an exothermic member of a heater wire for calibrating GT assembly 300. Differential-type thermocouples 314 may function as temperature sensors around built-in heater 312. Spaces within cladding tube 316 that are not occupied by built-in heater 312 or differential-type thermocouples 314 may be filled with electric insulating layer or metal/metal-alloy filler 318. Built-in heaters 312 may include cladding tubes 320, electric insulating layers 322, and/or heater wires 324. Differential-type thermocouples 314 may include cladding tubes 326, electric insulating layers 328, and/or thermocouple signal wires 330.
GT assembly 300 may include gamma ray heating detectors 332 (i.e., GT detectors 332). GT detectors 332 may be fixed at an axial position of GT assembly 300 near corresponding annular space portions 306. Each GT detector 332 may include high-temperature point 334 (also known as the insulated or hot junction) and low-temperature point 336 (also known as the uninsulated or cold junction) of differential-type thermocouple 314. High-temperature point 334 may be near corresponding annular space portion 306. Low-temperature point 336 may be below or above corresponding annular space portion 306.
During steady-state operation, gamma ray flux may be proportional to thermal neutron flux. The gamma ray flux may deposit energy in the form of heat in structural elements of GT assembly 300, such as core tube 304. The deposited heat energy may be proportional to the gamma ray flux. Because the removal of heat energy from GT detector 332 in a vicinity of annular space portions 306 is relatively low while the removal of heat energy from GT detectors 332 not in a vicinity of annular space portions 306 is relatively high, a temperature difference may develop between high-temperature point 334 and low-temperature point 336 of differential-type thermocouple 314. This temperature difference may be detected as a voltage difference in differential-type thermocouple 314, may be proportional to the gamma ray flux and, thus, may be proportional to thermal neutron flux. Therefore, during steady-state operation, GT assembly 300 effectively may measure thermal neutron flux.
Characteristic values for GT detector 332 may include sensitivity S0 (in millivolts per watt per gram or mV/(W/g)) and/or alpha factor α (in 1/mV or mV−1).
Although typically written as S0, sensitivity S0 may be understood to be time-dependent and, thus, may be written as S0(t). Alpha factor α may represent a temperature dependence related to physical properties of the structural material of GT detector 332. Alpha factor α may be considered to have a constant value.
Due to exposure in the high neutron and/or gamma flux environment of core 200, sensitivity S0(t) generally may decrease over time. This decrease may be expressed using Equation (1) below, where S0(0)=a+b.S0(t)=a+b*exp(−λ*t)  (1)
As known to a PHOSITA, values for a, b, and λ may be predicted based on previous data and/or experience. As also known to a PHOSITA, values for a, b, and λ may be calculated and/or verified based on data recorded during GT calibrations.
As discussed above, when calibrating GT assembly 300, built-in heater 312 may function as an exothermic member, providing additional heating PH (in W/g). A relationship between sensitivity S0(t), alpha factor α, unheated output voltage U (in mV) of GT detector 332, heated output voltage U′ (in mV) of GT detector 332, and additional heating PH of GT detector 332 may be expressed using Equation (2) below.S0(t)={[U′/(1+α*U′)]−[U/(1+α*U)]}/PH  (2)
When not calibrating GT assembly 300, a relationship between sensitivity S0(t), alpha factor α, output voltage Uγ (in mV) of GT detector 332, and gamma ray heating value Wγ (in W/g) of GT detector 332 may be expressed using Equation (3) below.Uγ=S0(t)*(1+α*Uγ)*Wγ  (3)
Rearranging Equation (3) above may allow the calculation of gamma ray heating value Wγ using Equation (4) below.Wγ=Uγ/[S0(t)*(1+α*Uγ)]  (4)
FIG. 5 is a perspective view, partly broken away, showing an arrangement relationship of detectors of an in-core fixed nuclear instrumentation system of a related art power distribution monitoring system. FIG. 6 is a front view, partly broken away, showing the arrangement relationship of the detectors in FIG. 5.
As known to a PHOSITA, core 500 may include a large number of groups of four fuel assemblies 502. An in-core nuclear instrumentation system may include a plurality of in-core nuclear instrumentation assemblies 504. In-core nuclear instrumentation assemblies 504 may be disposed at corner water gap 506, surrounded by a group of four fuel assemblies 502. In-core nuclear instrumentation assemblies 504 may be disposed at different positions in core 500 from control rods 508.
In-core nuclear instrumentation assemblies 504 may include a thin and long nuclear instrumentation tube 510, LPRM detector assembly 512, and GT detector assembly 514.
LPRM detector assembly 512, housed in nuclear instrumentation tube 510, may function as a fixed neutron detection means. LPRM detector assembly 512 may include a plurality (e.g., four) of LPRM detectors 516. LPRM detectors 516 may be discretely arranged in an axial direction of core 500, at equal intervals L in nuclear instrumentation tube 510. LPRM detectors 516 may substantially cover an effective fuel length H (see FIG. 6) of core 500. Each LPRM detector 516 may be configured to measure neutron flux so as to generate a neutron flux signal (LPRM signal) according to the measured neutron flux. And each LPRM detector 516 may be electrically connected to an LPRM signal processing unit (not shown).
GT detector assembly 514, also housed in nuclear instrumentation tube 510, may function as a fixed gamma ray detection means. GT detector assembly 514 may include a plurality (e.g., eight) of GT detectors 332.
GT detectors 332 may be discretely arranged in an axial direction of core 500 in nuclear instrumentation tube 510. GT detectors 332 may substantially cover the effective fuel length H of core 500. Each GT detector 332 may be configured to measure gamma ray flux so as to generate a gamma ray flux signal (GT signal) according to the measured gamma ray flux. And each GT detector 332 may be electrically connected to a GT signal processing unit (not shown).
A large number of fuel rods (not shown) may be housed in channel box 518. Channel box 518 may be, for example, rectangular or cylindrical.
FIG. 7 is a block diagram showing schematically a structure of a reactor power distribution monitoring system of a BWR.
As known to a PHOSITA, reactor power distribution monitoring system 700 of a BWR may include an in-core fixed nuclear instrumentation system 702. In-core fixed nuclear instrumentation system 702 may have detectors, signal processing units, and process control computer 704 for monitoring an operating mode of the BWR and/or core performance.
Process control computer 704 may include, for example, central processing unit (“CPU”) 706, memory unit 708, input console 710, and/or display unit 712. CPU 706 may be electrically connected to memory unit 708, input console 710, and display unit 712 so as to enable communication between them.
Process control computer 704 may include a function for simulating a core power distribution of the BWR and/or a function for monitoring a core performance of the BWR according to the simulated core power distribution.
As shown in FIG. 7, core 500 may be housed in reactor pressure vessel 714. Reactor pressure vessel 714 may be housed in primary containment 716.
As discussed above, each LPRM detector 516 may be configured to measure neutron flux so as to generate a neutron flux signal (LPRM signal) according to the measured neutron flux. And each LPRM detector 516 may be electrically connected to LPRM signal processing unit 718 using signal cables 720 through penetration portion 722, forming power range neutron flux measuring system 724. LPRM signal processing unit 718 may include a computer having a CPU, a memory unit, and so on.
As known to a PHOSITA, LPRM signal processing unit 718 may be operative to perform, for example, analog-to-digital (“A/D”) conversion operations and/or gain processing operations of each LPRM signal S2 transmitted from each LPRM detector 516 so as to obtain digital LPRM data D2, and then to transmit digital LPRM data D2 to process control computer 704.
As discussed above, GT detector assembly 514 may be configured so that a plurality of GT detectors 332 may be discretely arranged in the axial direction of core 500. A gamma ray heating value may be measured by each GT detector 332. The number of GT detectors 332 should be the same as or more than the number of LPRM detectors 516. Each GT detector 332 may be electrically connected to GT signal processing unit 726 using signal cable 728 through penetration portion 730, forming GT power distribution measuring system 732.
As known to a PHOSITA, GT signal processing unit 726 may be configured to obtain digital GT data D1 using GT signals S1 outputted from GT detectors 332, as well as sensitivity S0 and alpha factor α of the respective GT detector 332. Digital GT data D1 may represent a gamma ray heating value in watts per gram of unit weight (W/g). GT signal processing unit 726 may be operative to transmit digital GT data D1 to process control computer 704.
In-core fixed nuclear instrumentation system 702 may include gamma ray thermometer heater control unit 734. Gamma ray thermometer heater control unit 734 may be electrically connected to each built-in heater 312 using power cables 746.
Core state data measuring device 736 may be provided in reactor pressure vessel 714 and/or primary system piping (not shown). Core state data measuring device 736 may provide core state data signal S3. Core state data signal S3 may include, for example, control rod pattern, core coolant flow rate, internal pressure of reactor pressure vessel 714, feed water flow rate, feed water temperature (e.g., core inlet coolant temperature), and so on. Core state data signal S3 may be used as various operating parameters indicative of a reactor operating mode (state) of the BWR.
A first part of core state data measuring device 736, inside reactor pressure vessel 714, may be connected to core state data processing unit 738 using signal cable 740 through penetration portion 742. A second part of core state data measuring device 736, outside reactor pressure vessel 714, may be connected using signal cable 740 to core state data processing unit 738. The first and/or second parts of core state data measuring device 736 may form process data measuring system 744.
As known to a PHOSITA, core state data processing unit 738 may be configured to obtain digital core state data D3 using core state data signal S3. Core state data processing unit 738 may be operative to transmit digital core state data D3 to process control computer 704.
CPU 706 may include nuclear instrumentation control process module 748 and/or power distribution simulation process module 750. Nuclear instrumentation control process module 748 may monitor and/or control in-core fixed nuclear instrumentation system 702.
As known to a PHOSITA, power distribution simulation process module 750 may correct the power distribution simulation result of nuclear instrumentation control process module 748, using digital GT data D1, digital LPRM data D2, and/or digital core state data D3, in order to obtain a core power distribution reflecting the actually measured data in core 500.
Memory unit 708 may include nuclear instrumentation control program module PM1, power distribution simulation program module PM2, and/or power distribution learning (adaptive) program module PM3. Power distribution simulation program module PM2 may include a physics model, such as a three-dimensional thermal-hydraulic simulation code.
Power distribution simulation process module 750 may simulate neutron flux distribution in core 500, may simulate power distribution in core 500, and/or may simulate margins with respect to one or more operational thermal limits (e.g., maximum linear heat generation rate (“MLHGR”) and/or minimum critical power ratio (“MCPR”)) using power distribution simulation program module PM2. Power distribution simulation process module 750 may be operative to correct the simulation results in order to obtain a core power distribution reflecting the actually measured core nuclear instrumentation data on the basis of power distribution learning (adaptive) program module PM3.
As discussed above, power distribution simulation process module 750 may correct the simulated results (neutron flux distribution and/or power distribution in core 500) stored in memory unit 708—according to inputted digital GT data D1, digital LPRM data D2, and/or digital core state data D3—in order to determine an accurate core power distribution and/or an accurate margin with respect to the one or more operational thermal limits, which reflect the actual core nuclear instrumentation data (digital GT data D1, digital LPRM data D2, and/or digital core state data D3).
As known to a PHOSITA, LPRM detectors generally may include a cathode having fissionable material coated on the cathode. The fissionable material may be a mixture of U234 and U235. The U235 may be used to provide a signal proportional to the thermal neutron flux. But due to the extremely high thermal neutron flux in the nuclear reactor core, the U235 may be subject to burnout, which may cause the LPRM detector reading corresponding to a constant thermal neutron flux to gradually decrease over time. The U234 may absorb thermal neutrons to become U235, lengthening the life of the LPRM detector. Eventually, however, the LPRM detector reading corresponding to a constant thermal neutron flux may still gradually decrease over time.
As also known to a PHOSITA, a gamma thermometer may provide a capability to calibrate an associated LPRM detector. During steady-state operation, gamma flux may be proportional to thermal neutron flux. Thus, a gamma thermometer—located near the associated LPRM detector—may measure local gamma flux during a steady-state heat balance, as known to a PHOSITA. The local gamma flux may be related to the proportional thermal neutron flux and the associated LPRM detector may be calibrated based on the related proportional thermal neutron flux.
Various solutions to the problem of calibrating nuclear instruments in nuclear reactors—using gamma thermometers—have been proposed, as discussed, for example, in U.S. Pat. No. 4,614,635 (“the '635 patent”), U.S. Pat. No. 5,015,434 (“the '434 patent”), U.S. Pat. No. 5,116,567 (“the '567 patent”), and U.S. Pat. No. 5,204,053 (“the '053 patent”), as well as U.S. Patent Publication No. 2009/0135984 A1 (“the '984 publication”). The disclosures of the '635 patent, the '434 patent, the '567 patent, the '053 patent, and the '984 publication are incorporated in the present application by reference. However, these various solutions do not include calibrating nuclear instruments in nuclear reactors—using gamma thermometers—wherein the calibrating of the nuclear instruments may be performed simply, automatically, in real-time, and/or with reduced cost when the associated nuclear reactor is not in steady-state operation.
FIG. 8 is a block diagram of a related art GT signal processor 800 of a BWR.
As known to a PHOSITA and as discussed, for example, in Japanese Laid-Open Patent Publication No. 2001-083280 (“JP '280”)—and its associated machine translation—GT signal processor 800 of a BWR may include GT signal site board 802, GT control panel 804, and/or a communication circuit (via transmitter 834 and optical cable 836) between GT signal site board 802 and GT control panel 804. The equations and associated explanations of JP '280 and its associated machine translation are incorporated herein by reference.
GT signal site board 802 may include amplifiers 806, low-pass filters 808, multiplexer 810, A/D converter 812, signal holding circuit 814, digital signal processor (“DSP”) 816, memory 818, and/or input/output (“I/O”) buffer 820. Delayed gamma compensation module 822 may include signal holding circuit 814, digital signal processor (“DSP”) 816, and/or memory 818.
GT control panel 804 may include transmitter 824, CPU 826, I/O buffer 828, memory 830, and/or display console 832.
FIG. 8 also depicts transmitter 834, optical cable 836, GT heater control panel 838, I/O machine 840, heater wires 842, additional GT signal site board or boards 844, and/or differential thermocouples 846.
Delayed gamma compensation module 822 of GT signal processor 800 may disclose calibrating nuclear instruments in nuclear reactors—using gamma thermometers—wherein the calibrating of the nuclear instruments may be performed when the associated nuclear reactor is not in steady-state operation.
Delayed gamma compensation module 822 may define a total GT signal R(t) (in mV) to include a prompt component [a0*P(t)] (in mV) and a delayed component [Σ am*um(t), where in the summation Σ, m=1, 2, . . . , M] (in mV), as shown in Equation (5) below.R(t)=a0*P(t)+Σ(am*um(t))  (5)
In Equation (5), a0 may represent a constant term, P(t) may represent an instant response term, am may represent constant terms, and um(t) may represent a delayed response term, defined by Equation (6) below.um(t)=1/τm*∫P(t′)*exp[−(t−t′)/τm]dt′  (6)
The integration in Equation (6) may be performed from t=−∞ to t, and τm may be a thermal time constant.
Rearranging Equation (5) above may allow the calculation of instant response term P(t) using Equation (7) below.P(t)=[R(t)−Σ(am*um(t))]/a0  (7)
As known to a PHOSITA, instant response term P(t) for a given GT detector 332 may be converted to digital GT data D1 and then compared to digital LPRM data D2 for the purpose of calibrating a corresponding LPRM detector 516.